Graphite made of carbon is extremely important as an industrial material because of its extraordinary heat resistance and chemical resistance, and because it is highly electrically conductive and heat conductive. Although natural graphite is also used, industrially manufactured graphite is the most common.
For example, a method which is used industrially is to manufacture sheets of graphite by baking treatment of a polyimide or other aromatic polymer which is used as the starting raw material (Japanese Unexamined Patent Publication No. H4-84600). Applications for this artificial graphite include for example optical parts for x-rays, high-heat conduction sheets, diaphragms with excellent high-frequency characteristics and the like.
Efforts have been made in recent years to use carbon materials as emissive materials, and research has been directed at improving the electron emission characteristics thereof. A method which has been proposed using shape is one which employs pointed shapes such as carbon nanotubes. Methods in which the surface condition is altered include efforts to improve the electron emission characteristics by decreasing the work function.
A method which is known for decreasing work function is to cover the surface of a carbon formed around a core of fine metal particles with an alkaline metal or alkaline earth metal (Japanese Unexamined Patent Publication No. H10-188778).
In addition, in order for a carbon material to be used as a heat-conduction sheet or the like it must have such characteristics as flexibility, strength and the like. To this end it is necessary to control the foaming condition (porosity). One known method for manufacturing such a graphite sheet is a method for manufacturing a graphite sheet which is flexible, elastic and sufficiently thick by adding an inorganic or organic filler to the aromatic film which is the raw material and treating them at high temperature to achieve a uniform foaming condition (Japanese Unexamined Patent Publication No. 2000-44220).
Thus, efforts have been made to apply artificially-made graphite to a variety of uses, and it has been found that forming pores inside graphite while also introducing suitable metals therein is an effective means for further improving the flexibility, electron emission characteristics and the like of graphite material.
In contrast, the method disclosed in Japanese Unexamined Patent Publication No. H4-84600 is a method for manufacturing a graphite diaphragm by pressure molding at temperatures of 2000° C. or more in the step of heat treating a polymer film. In this method, however, there is no foaming, and the inside of the graphite cannot be provided with pores. Consequently, it is difficult to manufacture an oriented graphite with superior electron emission characteristics, flexibility, toughness and the like.
On the other hand, the method disclosed in Japanese Patent Laid-open Publication No. H10-188778 is a method of improving the electron emission characteristics by covering the surface of a carbon formed around a core of fine metal particles with an alkaline metal or alkaline earth metal in order to change the surface condition. In this method, however, the electron emission characteristics are improved by altering the surface condition. That is, this method is limited in its ability to improve the electron transmission characteristics because it does not form pores inside the graphite.
In the method disclosed in Japanese Unexamined Patent Publication No. 2000-44220, on the other hand, a situation was studied in which 3% by weight calcium stearate and 5% by weight calcium hydrogenphosphate were included in a baking raw material comprising filler with the aim of creating a uniform foaming condition by heat treating at high temperatures with the addition of an inorganic or organic filler to a raw material aromatic film. In order to control the foaming condition it is important to investigate amounts and types of filler. However, in this method metal elements were not investigated as filler, and improvement due to the catalytic effect of metal elements cannot be expected.
As described above, because it is difficult to control the foaming condition with conventional methods it is also difficult to provide the desired pores in the interior of the graphite.